1. Field of the Invention
The present invention relates to electrostatic reproducing apparatus.
2. Description of the Prior Art
A known electrostatic reproducing apparatus includes a charge receptor on which an electrostatic latent image is formed in accordance with the imaging signal of an image to be recorded. In this apparatus, after the latent image has been developed into a visible image, the latent image is not extinguished but is preserved on the charge receptor to permit only the visible image to be transferred to a sheet of recording paper; this step may then be repeated to form a plurality of reproduced images, or copies, with a single electrostatic image. Such apparatus is generally referred to as a "retention type electrostatic reproducing apparatus". One apparatus of this type incorporates a transfer belt to which the developed visible image on the charge receptor is transferred, the visible image then being further transferred to the recording paper and fixed thereon.
This retention type electrostatic reproducing apparatus, however, has the following disadvantage. The surface potential of the electrostatic latent image is gradually attenuated as the number of copies therefrom increases, as shown in FIG. 1. (Note: FIG. 1 shows the characteristics with parameters of optical density (O.D.), the illustrated optical density values of 0 and 1.3 being shown by way of example only.) As a consequence, the density of the reproduced image gradually decreases as shown in FIG. 2. This phenomenon is attributable to the following:
(1) As a copying operation is repeated, the electrostatic charge of the latent image gradually leaks through the body of the charge receptor.
(2) The electrostatic charge is neutralized by the charge produced by friction between the toner and the charge receptor. In addition, the electrostatic charge of the charge receptor leaks through the toner.
(3) In an apparatus having a transfer belt, the electrostatic charge of the latent image is negated by charges produced as the belt separates from the charge receptor.
As a consequence, the number of copies obtainable from a single electrostatic latent image is impractically limited. This problem is inherent in retention type electrostatic reproducing apparatus, and occurs in each of the operative steps including formation of the electrostatic latent image, development, transfer and fixing.